首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   18888篇
  免费   556篇
  国内免费   1233篇
化学   9432篇
晶体学   135篇
力学   110篇
综合类   56篇
数学   9561篇
物理学   1383篇
  2024年   13篇
  2023年   148篇
  2022年   210篇
  2021年   257篇
  2020年   356篇
  2019年   551篇
  2018年   520篇
  2017年   436篇
  2016年   370篇
  2015年   343篇
  2014年   658篇
  2013年   1742篇
  2012年   741篇
  2011年   1116篇
  2010年   903篇
  2009年   1109篇
  2008年   1260篇
  2007年   1294篇
  2006年   1077篇
  2005年   948篇
  2004年   806篇
  2003年   693篇
  2002年   476篇
  2001年   427篇
  2000年   390篇
  1999年   420篇
  1998年   400篇
  1997年   246篇
  1996年   324篇
  1995年   305篇
  1994年   340篇
  1993年   272篇
  1992年   232篇
  1991年   152篇
  1990年   137篇
  1989年   114篇
  1988年   88篇
  1987年   87篇
  1986年   59篇
  1985年   106篇
  1984年   95篇
  1983年   57篇
  1982年   89篇
  1981年   80篇
  1980年   52篇
  1979年   36篇
  1978年   41篇
  1977年   34篇
  1976年   34篇
  1974年   11篇
排序方式: 共有10000条查询结果,搜索用时 304 毫秒
1.
2.
A novel nanomagnetic basic catalyst of caesium carbonate supported on hydroxyapatite‐coated Ni0.5Zn0.5Fe2O4 magnetic nanoparticles (Ni0.5Zn0.5Fe2O4@HAP‐Cs2CO3) was prepared. This new catalyst was fully characterized using Fourier transform infrared spectroscopy, transmission and scanning electron microscopy, X‐ray diffraction and vibrating sample magnetometry techniques, and then the catalytic activity of this catalyst was investigated in the synthesis of 1H‐pyrazolo[1,2‐b]phthalazine‐5,10‐dione derivatives. Also, Ni0.5Zn0.5Fe2O4@HAP‐Cs2CO3 could be reused at least five times without significant loss of activity and could be recovered easily by applying an external magnet. Thus, the developed nanomagnetic catalyst is potentially useful for the green and economic production of organic compounds. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   
3.
We relate the distribution characters and the wave front sets of unitary representation for real reductive dual pairs of type I in the stable range.  相似文献   
4.
Let F be a field of characteristic 2. In this paper we give a complete computation of the kernel of the homomorphism H2m+1(F)?H2m+1(L) induced by scalar extension, where L/F is a purely inseparable extension (of any degree), H2m+1(F) is the cokernel of the Artin–Schreier operator ?:ΩFm?ΩFm/dΩFm?1 given by: xdx1x1?dxmxm?(x2?x)dx1x1?dxmxm+dΩFm?1, where ΩFm is the space of absolute m-differential forms over F and d is the differential operator. Other related results are included.  相似文献   
5.
A manifold that contains small perturbations will induce a perturbed partial differential equation. The partial differential equation that we select is the Poisson equation – in order to explore the interplay between the geometry of the manifold and the perturbations. Specifically, we show how the problem of symmetry determination, for higher-order perturbations, can be elegantly expressed via geometric conditions.  相似文献   
6.
We show the short-time existence and nonlinear stability of vortex sheets for the nonisentropic compressible Euler equations in two spatial dimensions, based on the weakly linear stability result of Morando and Trebeschi (2008) [20]. The missing normal derivatives are compensated through the equations of the linearized vorticity and entropy when deriving higher-order energy estimates. The proof of the resolution for this nonlinear problem follows from certain a priori tame estimates on the effective linear problem in the usual Sobolev spaces and a suitable Nash–Moser iteration scheme.  相似文献   
7.
This paper deals with the Cauchy–Dirichlet problem for the fractional Cahn–Hilliard equation. The main results consist of global (in time) existence of weak solutions, characterization of parabolic smoothing effects (implying under proper condition eventual boundedness of trajectories), and convergence of each solution to a (single) equilibrium. In particular, to prove the convergence result, a variant of the so-called ?ojasiewicz–Simon inequality is provided for the fractional Dirichlet Laplacian and (possibly) non-analytic (but C1) nonlinearities.  相似文献   
8.
《Tetrahedron letters》2019,60(24):1582-1586
Rh(II)-catalyzed decomposition of certain cyclic α-diazocarbonyl compounds in the presence of cyclic ethers has been shown to give bicyclic ring expansion products. These are thought to arise from a [1,4]-alkyl shift toward the carbonyl oxygen atom and are in contrast with the recently observed spirocyclic products of a Stevens-type [1,2]-alkyl shift within the postulated oxonium ylide intermediate. Quantum chemical calculations performed at the B3LYP/6-31G* level of theory showed that the former reaction pathway (toward fused bicycles) is kinetically preferred.  相似文献   
9.
10.
2 laser of wide tunability to pump 13CD3OD we could observe 10 new far infrared laser lines ranging from 109 μm to 472 μm. The lines were generated by pumping infrared absorbing transitions of large offset appertaining to the C-O stretching Q-branch. All lines were characterised in wavelength, polarisation, optimum pressure of operation and precise offset measurements. Received: 17 September 1996/Revised version: 18 November 1996  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号